Method and apparatus for performing handover in wireless communication system

ABSTRACT

The present disclosure relates to a pre-5 th -Generation (5G) or 5G communication system to be provided for supporting higher data rates Beyond 4 th -Generation (4G) communication system such as Long Term Evolution (LTE). 
     A handover method for a terminal according to one embodiment of the present disclosure may include: receiving, from a source cell, a message including information on one or more target cells for performing a fast handover; and performing a fast handover by reusing a radio bearer based on the received information.

TECHNICAL FIELD

Various embodiments of the present disclosure relate to a wirelesscommunication system, and more particularly, to a method and apparatusfor improving a handover procedure when a terminal moves between cells.

BACKGROUND ART

To meet the demand for wireless data traffic having increased sincedeployment of 4G communication systems, efforts have been made todevelop an improved 5G or pre-5G communication system. Therefore, the 5Gor pre-5G communication system is also called a ‘Beyond 4G Network’ or a‘Post LTE System’.

The 5G communication system is considered to be implemented in higherfrequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higherdata rates. To decrease propagation loss of the radio waves and increasethe transmission distance, the beamforming, massive multiple-inputmultiple-output (MIMO), Full Dimensional MIMO (FD-MIMO), array antenna,an analog beam forming, large scale antenna techniques are discussed in5G communication systems.

In addition, in 5G communication systems, development for system networkimprovement is under way based on advanced small cells, cloud RadioAccess Networks (RANs), ultra-dense networks, device-to-device (D2D)communication, wireless backhaul, moving network, cooperativecommunication, Coordinated Multi-Points (CoMP), reception-endinterference cancellation and the like.

In the 5G system, Hybrid FSK and QAM Modulation (FQAM) and slidingwindow superposition coding (SWSC) as an advanced coding modulation(ACM), and filter bank multi carrier (FBMC), non-orthogonal multipleaccess (NOMA), and sparse code multiple access (SCMA) as an advancedaccess technology have been developed.

Meanwhile, there is a need for a method for improving performance of aterminal by minimizing an inefficient process in a handover procedure inwhich the terminal moves between the cells.

DISCLOSURE OF INVENTION Technical Problem

An object of the present disclosure is directed to provision of a methodand an apparatus for reducing latency and performing handover withimproved performance by minimizing inefficient processes such as userplane reset or security key update which may occur during a handover ina wireless communication system.

Solution to Problem

Various embodiments of the present disclosure are directed to theprovision of a handover method of a terminal in a wireless communicationsystem, including: receiving, from a source cell, a message includinginformation on one or more target cell for performing a fast handover;and performing the fast handover by reusing a radio bearer based on thereceived information.

Various embodiments of the present disclosure are directed to theprovision of a handover method of a source cell in a wirelesscommunication system, including: receiving, from a terminal, ameasurement report including a measurement result for one or more cell;determining one or more target cell performing a fast handover reusing aradio bearer based on the measurement report; and transmittinginformation on the one or more target cell determined to a terminal.

Various embodiments of the present disclosure are directed to theprovision of a handover method of a target cell in a wirelesscommunication system, including: receiving, from a terminal, a handovermessage including a new terminal identifier in case that a fast handoveris completed; and reusing a security key corresponding to a source celland radio bearers of a specific user plane protocol such as service dataadaptation protocol (SDAP), packet data convergence protocol (PDCP),radio link control (RLC), a medium access control, or a physical layerbased on the new terminal identifier of the received message.

Advantageous Effects of Invention

According to the embodiment of the present disclosure, the terminal canreduce the latency and perform the handover with improved performance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sequence diagram illustrating a UE-based handover procedureaccording to a general method.

FIG. 2 is a sequence diagram illustrating a network-based handoverprocedure according to a general method.

FIG. 3 is a sequence diagram illustrating an inter-cell handoverprocedure in a UE-based base station according to an embodiment of thepresent disclosure.

FIG. 4 is a sequence diagram illustrating a UE-based inter-base stationhandover procedure according to an embodiment of the present disclosure.

FIG. 5 is a flowchart illustrating a UE-based handover procedure of aterminal according to an embodiment of the present disclosure.

FIG. 6 is a flowchart illustrating an inter-cell handover procedure in aUE-based base station of a source cell according to an embodiment of thepresent disclosure.

FIG. 7 is a flowchart illustrating a UE-based inter-base stationhandover procedure of a base station according to an embodiment of thepresent disclosure.

FIG. 8 is a sequence diagram illustrating an inter-cell handoverprocedure in a network-based base station according to an embodiment ofthe present disclosure.

FIG. 9 is a sequence diagram illustrating a network-based inter-basestation handover procedure according to an embodiment of the presentdisclosure.

FIG. 10 is a flowchart illustrating a network-based handover procedureof a terminal according to an embodiment of the present disclosure.

FIG. 11 is a flowchart illustrating an inter-cell handover procedure ina network-based base station of a base station according to anembodiment of the present disclosure.

FIG. 12 is a flowchart illustrating a network-based inter-base stationhandover procedure of a base station according to an embodiment of thepresent disclosure.

FIG. 13 is a block diagram illustrating a configuration of a terminalaccording to an embodiment of the present disclosure.

FIG. 14 is a block diagram illustrating a configuration of a basestation according to an embodiment of the present disclosure.

MODE FOR THE INVENTION

Hereinafter, embodiments of the present disclosure will be described indetail with reference to the accompanying drawings.

In describing the exemplary embodiments of the present disclosure in thepresent specification, a description of technical contents which arewell known to the art to which the present disclosure belongs and arenot directly connected with the present disclosure will be omitted. Thisis to more clearly transfer a gist of the present disclosure by omittingan unnecessary description.

For the same reason, some components are exaggerated, omitted, orschematically illustrated in the accompanying drawings. Further, thesize of each component does not exactly reflect its real size. In eachdrawing, the same or corresponding components are denoted by the samereference numerals.

Various advantages and features of the present disclosure and methodsaccomplishing the same will become apparent from the following detaileddescription of embodiments with reference to the accompanying drawings.However, the present disclosure is not limited to the embodimentsdisclosed herein but will be implemented in various forms. Theembodiments have made disclosure of the present disclosure complete andare provided so that those skilled in the art can easily understand thescope of the present disclosure. Therefore, the present disclosure willbe defined by the scope of the appended claims. Like reference numeralsthroughout the description denote like elements.

Further, it may be understood that each block of processing flow chartsand combinations of flow charts may be performed by computer programinstructions. Since these computer program instructions may be mountedin processors for a general computer, a special computer, or otherprogrammable data processing apparatuses, these instructions performedby the processors for the computer or the other programmable dataprocessing apparatuses create means performing functions described inblock(s) of the flow charts. Since these computer program instructionsmay also be stored in a computer usable or computer readable memory of acomputer or other programmable data processing apparatuses in order toimplement the functions in a specific scheme, the computer programinstructions stored in the computer usable or computer readable memorymay also produce manufacturing articles including instruction meansperforming the functions described in block(s) of the flow charts. Sincethe computer program instructions may also be mounted on the computer orthe other programmable data processing apparatuses, the instructionsperforming a series of operation steps on the computer or the otherprogrammable data processing apparatuses to create processes performedby the computer to thereby perform the computer or the otherprogrammable data processing apparatuses may also provide steps forperforming the functions described in block(s) of the flow charts.

In addition, each block may indicate some of modules, segments, or codesincluding one or more executable instructions for performing a specificlogical function(s). Further, it is to be noted that functions mentionedin the blocks occur regardless of a sequence in some alternativeembodiments. For example, two blocks that are consecutively illustratedmay be substantially simultaneously performed in fact or be performed ina reverse sequence depending on corresponding functions sometimes.

Here, the term ‘˜unit’ used in the present embodiment means software orhardware components such as FPGA and ASIC and the ‘˜unit’ performs anyroles. However, the meaning of the ‘˜unit’ is not limited to software orhardware. The ‘˜unit’ may be configured to be in a storage medium thatmay be addressed and may also be configured to reproduce one or moreprocessors. Accordingly, for example, the ‘˜unit’ includes componentssuch as software components, object oriented software components, classcomponents, and task components and processors, functions, attributes,procedures, subroutines, segments of program code, drivers, firmware,microcode, circuit, data, database, data structures, tables, arrays, andvariables. The functions provided in the components and the ‘˜units’ maybe combined with a smaller number of components and the ‘˜units’ or maybe further separated into additional components and ‘˜units’. Inaddition, the components and the ‘˜units’ may also be implemented toreproduce one or more CPUs within a device or a security multimediacard.

The terminal according to the embodiment of the present disclosure maygenerally include a mobile terminal and may indicate a device that ispreviously joined in a mobile communication system to receive a servicefrom the mobile communication system. The mobile terminal may includesmart devices such as a smart phone and a tablet PC, which is merely anexample, and the present disclosure is not limited thereto.

A cellular wireless communication system performs a handover procedurewhen a serving cell of a mobile terminal needs to be changed. Forexample, when a terminal that has performed communication with a sourcecell has difficulty in performing communication due to a disconnectioncaused by an obstacle, or when the terminal moves from a region in asource cell to a region in a target cell, a handover to the target cellmay be executed.

Hereinafter, a general handover procedure of a terminal will bedescribed with reference to FIGS. 1 and 2.

FIG. 1 is a sequence diagram illustrating a general UE-based handovermethod. More specifically, a process of performing, by a terminal 100, ahandover from a source cell 200 to a target cell 300 is illustrated. Thesource cell 200 and the target cell 300 may be cells for one basestation or cells of different base stations.

First, in step S110, the terminal 100 may transmit a measurement reportto the source cell 200. For example, the terminal 100 connected to thesource cell 200 and performing communication may periodically measure asignal level and report the measured signal level to the source cell200. Specifically, the terminal 100 may be set for a predeterminedcondition or a reporting period for the measurement report. According tothe setup, the terminal 100 may perform the measurement report whensatisfying the predetermined condition, or according to a predeterminedreporting period.

If the source cell 200 and the target cell 300 are cells of differentbase stations (for example, inter-handover), then in step S115, thesource cell 200 transmits a pre-handover request message to the targetcell 300. In step S120, the target cell 300 may transmit acknowledgementto the received pre-handover request message.

For example, the source cell 200 may request the UE-based handover tothe target cell 300 in advance through the pre-handover request message.If the target cell 300 receiving the pre-request may accommodate thehandover, the target cell 300 may transmit an Ack message for thepre-handover request to the source cell 200 to accept the handover.

Meanwhile, in the steps S115 and S120, the case in which the inter-cellhandover in the base station is performed may be omitted. For example,when the source cell 200 and the target cell 300 are cells of one basestation, it is not necessary to transmit and receive the pre-handoverrequest message and acknowledgement based on the measurement report ofthe terminal between the cells.

In step S125, the source cell 200 may provide, to the terminal 100,candidate cell information that the terminal 100 may be used in thehandover determination, through an RRC connection reconfigurationmessage.

For example, as described above, one or more target cell that transmitsthe Ack message for the pre-handover request may be a candidate cell.Accordingly, the source cell 200 may transmit the RRC connectionreconfiguration message which includes information on one or more targetcell that transmits an Ack message for the pre-handover request. Forexample, the source cell 200 may transmit a cell ID as the informationon the target cell.

In step S130, when the handover event is triggered, the terminal 100 maydetermine handover. For example, the terminal 100 may determine a targetcell performing a handover based on the candidate cell information.

If the handover to the target cell 300 is determined by the terminal100, the terminal 100 may perform an initial connection with the targetcell 300 through a random access procedure.

For example, in step S135, the terminal 100 may transmit a random accesspreamble to the target cell 300. As a response to the random accesspreamble, the terminal 100 may receive a random access response messagefrom the target cell 300 in step S140.

In step S145, the terminal 100 may transmit an RRC connectionreestablishment request message to the target cell 300 and in step S150,receive an RRC connection reestablishment message.

By the above-mentioned process, in step S155 and step S160, the terminal100 and the target cell 300 that the connection is completed may eachrelease and then re-establish radio bearers of service data adaptationprotocol (SDAP), packet data convergence protocol (PDCP), a radio linkcontrol (RLC), a medium access control, and a physical layer.

Specifically, the terminal 100 and the target cell 300 each reset apacket data unit (PDU) existing in a user plane protocol stack (forexample, RLC and PDCP).

In step S165 and step S170, the terminal 100 and the target cell 300 mayeach update a security key.

For example, the source cell 200 may update the security key using aphysical cell ID (PCID) of the target cell 300 and transmit the updatedsecurity key to the target cell 300. Accordingly, the target cell 300receiving the updated security key may use the updated security key toencrypt or decrypt the transmitted/received data.

In addition, the source cell 200 may transmit parameters for updatingthe secret key from the terminal 100. The terminal 100 may generate theupdated security key using the information received from the source cell200. The terminal 100 that has completed the handover procedure mayencrypt or decrypt data transmitted and received using the updatedsecurity key.

In step S175, the terminal 100 may transmit an RRC connectionreestablishment complete message to the target cell 300.

Meanwhile, FIG. 2 is a sequence diagram illustrating a network-based(NW-based) handover procedure according to a general method. In FIG. 2,a difference from the UE-based handover procedure described withreference to FIG. 1 will be mainly described.

First, in step S210, the terminal 100 may transmit a measurement reportto the source cell 200.

In step S215, the source cell 200 may determine a handover. For example,the source cell 200 may determine a target cell performing a handoverbased on the measurement report received from the terminal 100.

If the source cell 200 and the target cell 300 each are cells ofdifferent base stations, in step S220, the source cell 200 may transmita handover request to the target cell 300. At this time, the source cell200 may transmit the updated security key based on the physical cell ID(PCID) of the target cell 300.

In step S225, the target cell 300 which determines that the handover isacceptable may transmit a handover request response to the source cell200.

In the steps S220 and S225, since the source cell 200 and the targetcell 300 each are cells of different base stations, it is a necessaryprocedure only when an inter-base station handover is performed.Accordingly, when the source cell 200 and the target cell 300 are cellsof the same base station, the above steps S220 and S225 may be omitted.

In step S230, the source cell 200 receiving the handover requestresponse may transmit an RRC connection reconfiguration messageincluding mobility control information to the terminal 100. At thistime, the source cell 200 may transmit the information on the targetcell 300 to be handed over to the terminal 100.

Meanwhile, if the source cell 200 and the target cell 300 each are cellsof different base stations, in step S235, the source cell 200 maytransmit a sequence number status (SNstatus) to the target cell 300.Specifically, the source cell 200 may transmit a sequence number of apacket of a PDCP layer to the target cell 300 through an interfacebetween the base stations, and inform from which packet is to betransmitted and received to the terminal 100 after the handover iscompleted.

In the step S235, since the source cell 200 and the target cell 300 eachare cells of different base stations, it is a necessary procedure onlywhen the inter-base station handover is performed. Accordingly, when thesource cell 200 and the target cell 300 are cells of the same basestation, the above step S235 may be omitted.

Meanwhile, in step S240 and step S245, the terminal 100 and the targetcell 300 may each reset radio bearers (RB) of service data adaptationprotocol (SDAP), packet data convergence protocol (PDCP), radio linkcontrol (RLC), medium access control, and a physical layer.

In step S250 and step S255, the terminal 100 and the target cell 300 mayeach update a security key.

Meanwhile, in step S260, the terminal 100 may transmit the random accesspreamble to the target cell 300. As the response to the random accesspreamble, the terminal 100 may receive a random access response messagefrom the target cell 300 in step S265.

In step S270, the terminal 100 may transmit the RRC connectionreconfiguration complete message to the target cell 300 to complete theconnection with the target cell 300.

In the UE-based and NW-based handover processes as shown in FIGS. 1 and2, a radio bearer established in the source cell may be reset andre-established in the target cell. By the re-establishment, user data ofthe user plane protocol stack (for example, PDCP and RLC) may also bereset.

Further, when the cell is changed through the handover, the security keyis updated. Therefore, even if the user data encrypted using thesecurity key in the PDCP layer is transmitted to the target cell, theupdated security key can not be decrypted.

On the other hand, when communication is performed in a high frequencyband (for example, 28 GHz) as in the 5G system, the case in which theterminal is handed over to the target cell may be frequent for thereason that the communication with the source cell is impossible due toa deterioration in radio environment caused by a temporary obstaclebetween the base station and the terminal which provide thecommunication service.

At this time, the handover performance is important, and one of theperformance factors may be a handover setup time. However, as describedabove, generally, the security key should be changed every time the cellis changed, and the existing user data should be reset, which is aninefficient structure. Therefore, there is a problem that it isdifficult to perform a fast handover.

Hereinafter, a handover method for solving the above problems will bedescribed in detail.

FIG. 3 is a sequence diagram illustrating an inter-cell handoverprocedure in a UE-based base station according to an embodiment of thepresent disclosure.

First, in step S310, the terminal 100 may transmit the measurementreport to the source cell 200.

In step S315, the source cell 200 may transmit the RRC connectionreconfiguration message to the terminal 100. At this time, the sourcecell 200 may provide, to the terminal 100, the candidate cellinformation that the terminal 100 may be used in the handoverdetermination, through an RRC connection reconfiguration message.

For example, the source cell 200 may transmit, to the terminal 100, theinformation on the candidate cells that the terminal 100 may handoverbased on the measurement report. Among the candidate cells, one or morecandidate cell capable of performing the fast HO may be provided.

Accordingly, the source cell 200 may transmit, as information on acandidate cell capable of performing the fast HO, the candidate cellinformation including a new terminal ID (for example, identifier(C-RNTI) which the target cell allocates) for the fast handover(hereinafter, fast HO), system information (SI) on the target cell, RACHinformation, and user protocol information to be reused according to acentral unit (CU)-access unit (AU) function split option. In the fastHO, the user protocol configuration to be specified and reused in thetarget cell may include, for example, the following combinations.

-   -   Reuse only PDCP protocol    -   Reuse only PDCP, RLC protocols    -   Reuse only PDCP, RLC, MAC protocols    -   Reuse only PDCP, RLC, MAC, PHY protocols    -   Reuse only SDAP, PDCP protocols    -   Reuse only SDAP, PDCP, RLC protocols    -   Reuse only SDAP, PDCP, RLC, MAC protocols    -   Reuse only SDAP, PDCP, RLC, MAC, PHY protocols

Specific examples of the candidate cell information are shown in Table 1as follows.

TABLE 1 Candidate Cell Info Description Option Condition candidateCellIDPhysical Cell ID of Mandatory Essential in UE- Candidate Cell based HO(Apply for both normal and fast HOs) newUE-Identity C-RNTI of OPTIONALEssential in UE- Candidate Cell based fast HO radioResourceConfigCommonCommon radio OPTIONAL Essential in UE- resource based fast HOconfigurations of Candidate Cell rach- Dedicated random OPTIONALSelective in UE- ConfigDedicated access parameters based fast HO ofCandidate Cell ReuseProtocolType Target protocols Mandatory Essential inUE- (SDAP, PDCP, based HO (Apply RLC, MAC, PHY) for both normal and tobe reused fast HOs) without being reset in fast handover

Meanwhile, in step S320, when the handover event is triggered and thetarget cell is determined, the terminal 100 may determine whether it isthe fast HO based on the candidate cell information on the determinedtarget cell. For example, when performing handover between cells in abase station, the terminal 100 may be preset to perform the fast HO.Therefore, the terminal 100 may determine to execute the fast HO on thecell of the base station, such as the source cell 200, based on thecandidate cell information.

In the case of the fast HO, the terminal does not reset the specificuser plane protocols (that is, PDCP, RLC, MAC and PHY) according to areuse protocol application target, and may also maintain the securitykey as a key used in the source cell.

The terminal 100 may perform a dedicated random access or acontention-based random access according to whether rach-ConfigDedicatedis configured in the candidate cell information in order to perform therandom access procedure with the target cell.

Specifically, the terminal 100 having determined to perform the fast HOto the target cell 300 may transmit the random access preamble to thetarget cell 300 in step S325. As the response to the random accesspreamble, in step S330, the terminal 100 may receive the random accessresponse message from the target cell 300.

When the random access procedure is completed, in step S335, theterminal 100 may transmit the RRC connection reconfiguration completemessage including the C-RNTI information on the candidate cell as a newterminal identifier to the target cell 300.

Accordingly, the target cell 300 may reuse the specific user planeprotocol (e.g., PDCP/RLC) release of the source cell 200 and thesecurity key without resetting the specific user plane protocol (e.g.,PDCP/RLC) release of the source cell 200 and the security key using thereceived new terminal identifier. For example, the target cell 300 mayuse the security key of the source cell as it is. The target cell 300may not perform the resetting of the specific user plane protocol (e.g.,PDCP/RLC) release.

Meanwhile, FIG. 4 is a sequence diagram illustrating a UE-basedinter-base station handover procedure according to an embodiment of thepresent disclosure. Unlike FIG. 3, even when the handover between cellsof different base stations is performed, the fast HO may be performedaccording to predetermined conditions. For example, when a service usedby the terminal is a service (for example, a streaming service) thatneeds to transmit and receive data without interruption or when thereare many obstacles around the terminal or the handover is frequentlyrepeated at any time, it may be determined that the fast HO condition issatisfied.

First, the source cell 200 receiving the measurement report from theterminal 100 in step S410 may transmit a pre-handover request message tothe target cell 300 in step S415. In step S420, the target cell 300 maytransmit the acknowledgement to the received pre-handover requestmessage.

Specifically, the source cell 200 may transmit the pre-handover requestmessage including information on the RB of the user plane (e.g.,PDCP/RLC) and the security key of the source cell 200. Accordingly, thetarget cell 300 may use the security key of the source cell 200 when thefast HO is performed by the determination of the terminal 100.

If the target cell 300 receiving the pre-request may accommodate thehandover, the target cell 300 may transmit an Ack message for thepre-handover request to the source cell 200 to accept the handover.

In step S425, the source cell 200 may provide, to the terminal 100,candidate cell information that the terminal 100 may be used in thehandover determination, through an RRC connection reconfigurationmessage. For example, the source cell 200 may transmit the candidatecell information including the new terminal ID (for example, identifier(C-RNTI) that the target cell allocates) for the fast HO, the target SI,and the RACH information.

In step S430, the terminal 100 may determine a handover. In detail, whenthe handover event is triggered and the target cell is determined, theterminal 100 may determine whether it is the fast HO based on thecandidate cell information on the determined target cell.

The terminal 100 in which the fast HO is determined by the target cell300 does not reset the specific user plane protocol (for example, PDCPand RLC) and may also maintain the security key as a key used in thesource cell.

In addition, in steps S435 to S445, the random access procedure may beperformed and the RRC connection reconfiguration complete message may betransmitted. The detailed process of steps S430 to S445 is as describedin FIG. 3.

Accordingly, the target cell 300 may also reuse the RB of the specificuser plane protocol (e.g., PDCP/RLC) of the source cell 200 and thesecurity key without resetting the RB of the specific user planeprotocol (e.g., PDCP/RLC) and the security key.

Meanwhile, in step S450, the target cell 300 may transmit a fast HOindication to the source cell 200. Specifically, since the target cell300 is a cell of a base station different from the source cell 200, thetarget cell 300 may receive the existing RLC RB by transmitting the fastHO indication.

Accordingly, the source cell 200 receiving the fast HO indication maytransmit the SNstatus of the RLC to the target cell 300 in step S455.Therefore, the target cell 300 may reuse the existing RLC RB withoutresetting the RLC RB.

The source cell 200 may forward, to the target cell, the RLC packethaving the SN following the SN of the data packet transmitted from thesource cell to the terminal according to the SNstatus of the RLC.Hereinafter, specific operations of the terminal and the base station inthe UE-based fast HO as described above with reference to FIGS. 5 to 7will be described.

Specifically, FIG. 5 is a flowchart illustrating a UE-based fast HOprocedure of a terminal according to an embodiment of the presentdisclosure. When the terminal performs the UE-based fast HO, it mayoperate regardless of intra or inter HO.

In step S500, the terminal may transmit the measurement report (MR) tothe source cell.

For example, the base station may configure the measurement report tothe terminal to select the candidate cell for UE-based handover(hereinafter, referred to as the UE-based HO). Accordingly, the terminalmay transmit the measurement report to the source cell.

In step S510, the terminal may receive the candidate target cellinformation for the UE-based HO. Specifically, the base station mayselect one or more candidate cells that may be the target cell at thetime of UE-based HO based on the MR, and transmits the candidate cellinformation to the terminal. The attribute information on the candidatecell may include a new terminal identifier (newUE-Identity) including acandidate cell ID, a radio resource configuration command(radioResourceConfigCommon) information, RACH information(rach-ConfigDedicated), and the like.

On the other hand, when the fast HO is not performed, the source cellmay transmit the candidate cell information including only the candidatecell identifier to the terminal.

In step S520, the terminal may determine a handover when a UE-based HOevent trigger occurs. In step S530, the terminal may determine whetherthe target cell performing the handover is the fast handover targetcell.

For example, if the handover event is triggered and the target cell isselected, the terminal may determine whether there is the fast HOoperation based on the candidate cell information on the target cell.For example, the terminal may receive information on candidate cells forthe UE-based OH from the base station. The base station may transmit thecandidate cell information including information necessary for the fastHO to the terminal when there is a cell necessary for the fast HO amongthe candidate cells (for example, when it is determined that other cellsin the base station among the candidate cells are required to be thefast HO, when the service used by the terminal is the streaming servicerequired to transmit and receive data without interruption, or when theoccurrence of the handover for the specific terminal or the specificcell, the occurrence of the failure of the handover or the radio linkfailure (RLF) is frequent).

At this time, the base station may transmit new UE-Identity,radioResourceConfigCommon, rach-ConfigDedicated, and ReuseProtocolTypeinformation as information required for the fast HO. In addition, asdescribed above, the base station may transmit only the informationnecessary for the fast HO or non-explicitly inform the terminal of thetarget cell performing the fast HO, while the base station mayexplicitly transmit the fast HO indication to the terminal.

Accordingly, the terminal may perform the fast HO when the handoverevent is triggered and the candidate cell information including theadditional information for the fast HO is included in the target cellperforming the handover.

If the performance of the fast HO is determined, in step S540, theterminal may reuse (no reset) the existing PDCP security key and theradio bearer (RB) of the specific user plane protocol (for example,PDCP/RLC).

Specifically, the terminal performing the fast HO does not reset the RBof the specific user plane protocol (for example, PDCP/RLC), and alsomaintains the security key as the security key used in the source cell.

In step S550, the terminal may perform the random access procedure withthe target cell. Specifically, the terminal may perform a dedicatedrandom access or a contention-based random access according to whetherthe rach-ConfigDedicated is configured in the received candidate cellinformation.

When the random access procedure is normally completed, the handovercomplete message including the C-RNTI information on the candidate cellmay be transmitted to the target base station. For example, the terminalmay transmit an RRC reconfiguration complete message to the target basestation.

On the other hand, in step S530, if the target cell is not the targetcell of the fast HO target, the terminal proceeds to step S570 and mayperform a general handover procedure. The general handover procedure isas described above with reference to FIGS. 1 and 2.

FIG. 6 is a flowchart illustrating an inter-cell handover procedure in aUE-based base station of a source cell according to an embodiment of thepresent disclosure.

First, in step S600, the source cell may receive a measurement reportfrom a terminal for selecting a candidate target cell.

For example, if the source cell determined that the UE-based HO isrequired, the source cell may transmit a measurement setting value tothe terminal for selecting a candidate cell. The source cell receivesthe measurement report from the terminal according to the set value.

In step S610, the source cell may transmit to the terminal the candidatetarget cell information for the UE-based HO.

For example, the source cell transmits to the terminal the informationon the candidate cell that may be a target cell at the time of theUE-based HO based on the measurement report information. Thecorresponding candidate cell information may include information forallowing the UE to recognize whether the fast HO is required for eachcandidate cell as described above with reference to Table 1.

In step S620, the terminal may perform the random access procedure withthe target cell. Specifically, if the handover event is triggered in theterminal and one of the candidate cells in the base station is selectedas the target cell, the terminal may perform a random access procedurewith the target cell.

In step S630, the target cell may determine whether a handover completemessage including a new terminal identifier for a fast handover isreceived from the terminal. If the handover complete message includingthe new terminal ID is received, in step S640, the target cell may reusea security key of the existing source cell and a radio bearer (RB) of aspecific user plane protocol (for example, PDCP/RLC).

For example, if the fast HO is determined by the terminal based on thecandidate cell information, the terminal may receive the handovercomplete message including the new terminal identifier for the fast HOto the target cell.

If the handover complete message including the new terminal identifieris received, the target cell may recognize that the terminal isperforming a handover through the fast HO. Therefore, the target cellmay reuse the security key of the existing source cell and the RB of thespecific user plane protocol (for example, PDCP/RLC) without resettingthe security key of the existing source cell and the RB of the specificuser plane protocol.

On the other hand, in step S630, if the target cell does not receive thehandover complete message including the new terminal identifier, forexample, when the target cell receives the RRC connectionreestablishment request message, the process proceeds to step S650, ageneral handover procedure is performed.

Meanwhile, FIG. 7 is a flowchart illustrating a UE-based inter-basestation handover (UE-based inter-HO) procedure of the base stationaccording to an embodiment of the present disclosure. For example, thesource cell may be a cell of a source base station, and the target cellmay be a cell of a target base station that is different from the sourcebase station.

First, in step S700, the source cell may receive a measurement reportfrom a terminal for selecting a candidate target cell.

For example, if the source cell is determined that the UE-based HO isrequired, the source cell may transmit a measurement setting value tothe terminal for selecting a candidate cell. The source cell receivesthe measurement report from the terminal according to the set value.

In step S710, the source cell may transmit the information on the radiobearer (RB) of the specific user plane protocol (for example, PDCP/RLC)and the security key to the target cell. For example, the target cellmay reuse the information on the RB of the received specific user plane(for example, PDCP/RLC) and the security key when the connection withthe UE is completed.

When the ack signal for the transmission is received from the targetcell in step S720, the source cell may transmit the candidate targetcell information for the UE-based handover to the terminal in step S730.

The terminal receiving the information on the target cell for the fastHO may perform the fast HO to the target cell by the source cell.Accordingly, the terminal may reuse the RB of the specific user planeprotocol (for example, PDCP/RLC) and the security key.

Meanwhile, in step S740, the terminal and the target cell of the targetbase station may perform the random access procedure. In step S750, thetarget cell may determine whether the handover complete messageincluding the new terminal identifier for the fast handover is receivedfrom the terminal.

For example, if the fast HO is performed, the terminal may transmit thehandover complete message including the new terminal identifier.Therefore, the target cell may determine whether to perform the fast HObased on whether the handover complete message including the newterminal ID is received.

Therefore, in step S760, the target cell may reuse the security key ofthe existing source cell and the RB of the specific user plane protocol(for example, PDCP/RLC).

In step S770, the target cell may transmit the fast HO indication to thesource cell. Specifically, since the target cell 300 is a cell of a basestation different from the source cell 200, the target cell 300 mayreceive the existing RLC RB by transmitting the fast HO indication.

Accordingly, in step S780, the source cell 200 receiving the fast HOindication may transmit the SNstatus of the RLC to the target cell 300.Therefore, the target cell 300 may reuse the existing RLC RB withoutresetting the RLC RB.

Meanwhile, if the handover completion message including the new terminalID for the fast HO is not received from the terminal in step S750, thetarget cell may perform the general handover procedure with the terminal(step S790).

Hereinafter, a procedure for performing the network-based (NW-based)handover will be described in detail.

FIG. 8 is a sequence diagram illustrating an NW-based intra HO procedureaccording to an embodiment of the present disclosure. Therefore, thesource cell 200 and the target cell 300 of FIG. 8 may be a cell of thesame base station.

First, in step S810, the terminal 100 may transmit a measurement reportto the source cell 200. For example, the terminal 100 connected to thesource cell 200 and performing communication may periodically measure asignal level and report the measured signal level to the source cell200.

Specifically, the terminal 100 may be set for a predetermined conditionor a reporting period for the measurement report. According to thesetup, the terminal 100 may perform the measurement report whensatisfying the predetermined condition, or according to a predeterminedreporting period.

In step S820, the source cell 200 receiving the measurement report maydetermine the handover. In the NW-based HO, unlike the UE-based HO, asubject determining the handover may be the source cell.

In detail, the source cell 200 may determine the target cell performingthe handover based on the measurement report that the terminal 100transmits. The source cell 200 may determine that the target cell 300should perform the fast HO. At this time, the source cell 200 may notperform the RB reset of the specific user plane protocol (for example,PDCP/RLC) and the security key update. For example, if the source cell200 is a cell within the same base station as the target cell, it maydetermine to perform the fast HO.

If the handover is determined, in step S830, the source cell 200 maytransmit the RRC connection reconfiguration message including themobility control information to the terminal 100. At this time, thesource cell 200 may transmit the information on the target cell 300 tobe handed over to the terminal 100. As described above, when the sourcecell 200 determines the fast HO with the target cell 300, the sourcecell 200 may transmit the fast HO indication information for the targetcell 300.

Therefore, the terminal 100 receiving the RRC connection reconfigurationmessage performs the fast HO, and thus can reuse the RB of the specificuser plane protocol (for example, PDCP/RLC) and the security key withoutresetting the RB of the specific user plane protocol and the securitykey. For example, the terminal 100 may use the security key with thetarget cell 300 as it is without updating the security key used with thesource cell 200.

In step S840, the terminal 100 may transmit the random access preambleto the target cell 300. As a response to the random access preamble, theterminal 100 may receive a random access response message from thetarget cell 300 in step S850.

In addition, in step S860, the terminal 100 may transmit the RRCconnection reconfiguration complete message to the target cell 300 tocomplete the connection with the target cell 300. At this time, theterminal 100 may transmit the RRC connection reconfiguration completemessage including the new terminal identifier.

Therefore, the target cell 300 receiving the new terminal identifier mayreuse the security key of the existing source cell and the RB of thespecific user plane protocol (for example, PDCP/RLC).

Meanwhile, FIG. 9 is a sequence diagram illustrating an NW-based interHO procedure according to an embodiment of the present disclosure.

First, in step S910, the terminal 100 may transmit the measurementreport to the source cell 200. For example, the terminal 100 connectedto the source cell 200 and performing communication may periodicallymeasure the signal level and report the measured signal level to thesource cell 200.

In step S920, the source cell 200 receiving the measurement report maydetermine the handover. For example, the source cell 200 may determinethe target cell and determine whether to perform the fast HO. If thetarget cell to be handed over satisfies a predetermined condition, thesource cell 200 may determine to perform the fast HO.

If the fast HO is determined, in step S930, the source cell 200 maytransmit the handover request to the target cell 300 to be handed over.At this time, the source cell 200 may transmit the information on the RBof the specific user plane protocol (for example, PDCP/RLC) and thesecurity key to the target cell 300.

In step S940, the target cell 300 which determines that the handover isacceptable may transmit a handover request response to the source cell200.

In step S950, the source cell 200 receiving the ack response maytransmit an RRC connection reconfiguration message including mobilitycontrol information to the terminal 100. At this time, the source cell200 may transmit the information on the target cell 300 to be handedover to the terminal 100. For example, the source cell 200 may transmitthe fast HO indication information for the target cell to the terminal100.

Therefore, the terminal 100 receiving the RRC connection reconfigurationmessage may reuse the RB of the specific user plane protocol (forexample, PDCP/RLC) and the security key.

Meanwhile, in step S960, the source cell 200 may transmit the SNstatusto the target cell 300. Specifically, the source cell 200 may transmit asequence number of a packet of a RLC layer to the target cell 300through an interface between the base stations, and inform from whichpacket is to be transmitted and received to the terminal 100 after thehandover is completed. The source cell 200 may forward, to the targetcell, the data packet having the SN following the SN of the data packettransmitted from the source cell to the terminal according to theSNstatus of the RLC.

The terminal 100 and the target cell 300 may perform the random accessprocedure. Specifically, in step S970, the terminal 100 may transmit therandom access preamble to the target cell 300. As the response to therandom access preamble, the terminal 100 may receive a random accessresponse message from the target cell 300 in step S980.

In addition, in step S990, the terminal 100 may transmit the RRCconnection reconfiguration complete message to the target cell 300 tocomplete the connection with the target cell 300. At this time, theterminal 100 may transmit the RRC connection reconfiguration completemessage including the new terminal identifier.

Therefore, the target cell 300 receiving the new terminal identifier mayreuse the security key of the existing source cell and the RB of thespecific user plane protocol (for example, PDCP/RLC).

Hereinafter, specific operations of the terminal and the base station inthe NW-based fast HO as described above with reference to FIGS. 10 to 12will be described.

Specifically, FIG. 10 is a flowchart illustrating the NW-based fast HOprocedure of a terminal according to an embodiment of the presentdisclosure. When the terminal performs the NW-based fast HO, it mayoperate regardless of intra or inter HO.

First, in step S1000, when the network-based handover event istriggered, in step S1010, the terminal may transmit the measurementreport to the source cell. In step S1020, the handover command messageincluding the fast handover indication information may be received fromthe source cell.

Specifically, the terminal transmits the measurement report to thesource cell due to the trigger of the HO event. The source cell maytransmit the HO command including the information on whether to performthe target cell and the fast HO to the terminal.

In step S1030, the terminal may determine whether the target cell is thefast HO target cell. For example, if the terminal receives theinformation required for the fast HO through the target cellinformation, the terminal may determine that the target cell is the fastHO target cell. According to an embodiment of the present disclosure,when the intra-cell HO in one base station is performed, the source cellmay be determined to satisfy the fast HO condition. Accordingly, theterminal receiving the target cell information including informationnecessary for the fast HO in the intra-HO may perform the fast HO on thetarget cell.

On the other hand, if the target cell is the fast HO target cell, instep S1040, the terminal does not reset the RB of the specific userplane protocol (for example, PDCP/RLC), and may also keep the securitykey as the key used in the source cell.

In step S1050, the terminal may perform the random access procedure withthe target cell.

When the handover procedure is completed, in step S1060, the terminalmay transmit the HO complete message (for example, RRC reconfigurationcomplete message) including the new terminal identifier to the targetcell.

In step S1030, if the target cell is not the fast HO, the terminalproceeds to step S1070 and may perform the general handover procedure.

Meanwhile, FIG. 11 is a flowchart illustrating the NW-based intra-HOprocedure of the base station according to an embodiment of the presentdisclosure.

First, in step S1100, the source cell may receive the measurement reportdue to the handover event trigger from the terminal. In step S1200, thesource cell may determine the handover. Specifically, the source cellmay determine the target cell which performs the handover based on thereceived measurement report.

In step S1120, the source cell may determine whether the fast HO isrequired for the determined target cell. For example, if a service usedby a terminal performing communication is a service such as streamingwhich needs to transmit and receive data seamlessly, the source cell maydetermine that the fast HO is required. Alternatively, if the terminaldetermines that the handover is repeated frequently for a certain time,the source cell may determine that the fast HO is required for theterminal.

As the determination result, if the fast HO is required for thedetermined target cell, in step S1130, the source cell may not performthe RB reset of the specific user plane protocol (for example, PDCP/RLC)and the update of the security key.

Therefore, in step S1140, the source cell may transmit to the HO commandmessage including the fast HO indication information to the terminal.

In step S1150, the random access procedure between the terminal and thetarget cell may be performed.

When the handover procedure is completed, in step S1160, the target cellmay receive the HO complete message (for example, RRC reconfigurationcomplete message) including the new terminal identifier from theterminal.

Therefore, in step S1170, the target cell may reuse the security key ofthe existing source cell and the radio bearer (RB) of the specific userplane protocol (for example, PDCP/RLC). For example, when the terminalperforms the fast HO through the new terminal identifier, the targetcell does not reset the security key and the radio bearer of thespecific user plane protocol (for example, PDCP/RLC).

On the other hand, in step S1120, if the fast HO is not required, theprocess proceeds to step S1180, and thus terminal and the target basestation may perform the general handover procedure.

FIG. 12 is a flowchart illustrating the NW-based inter-HO procedure ofthe base station according to an embodiment of the present disclosure.The NW-based inter-HO procedure shown in FIG. 12 is similar to theNW-based intra-HO procedure shown in FIG. 11 as described above.However, since the source cell and the target cell are cells ofdifferent base stations in FIG. 12, there may be added operations.

First, in step S1200, the source cell may receive the measurement reportdue to the handover event trigger from the terminal. In step S1210, thesource cell may determine the handover. Specifically, the source cellmay determine the target cell which performs the handover based on thereceived measurement report.

In step S1220, the source cell may determine whether the fast HO isrequired for the determined target cell.

As the determination result, if the fast HO is required for thedetermined target cell, in step S1230, the source cell may transmit theinformation on the RB of the specific user plane protocol (for example,PDCP/RLC) and the security key to the target cell of the target basestation.

The target cell receiving the information on the RB of the specific userplane protocol (for example, PDCP/RLC) and the security key may transmitthe ack signal to the source cell (step S1240).

Therefore, in step S1250, the source cell may transmit to the HO commandmessage including the fast HO indication information to the terminalAccordingly, when the terminal performs the handover to the target cellthat needs to perform the fast HO, it may reuse the RB of the specificUser Plane Protocol (for example, PDCP/RLC) and the security key.

Meanwhile, in step S1260, the source cell may transmit a sequence numberstatus (SN status) of the RLC to the target cell. Specifically, thesource cell may transmit a sequence number of a packet of a RLC layer tothe target cell through an interface between the base stations, andinform from which packet is to be transmitted and received to theterminal 100 after the handover is completed.

In step S1270, the random access procedure between the terminal and thetarget cell may be performed.

When the HO procedure is completed, in step S1280, the target cell mayreceive the HO complete message (for example, RRC reconfigurationcomplete message) including the new terminal identifier from theterminal.

Therefore, in step S1290, the target cell may reuse the security key ofthe existing source cell and the radio bearer (RB) of the specific userplane protocol (for example, PDCP/RLC). For example, when the terminalperforms the fast HO through the new terminal identifier, the targetcell does not reset the security key and the radio bearer of thespecific user plane protocol (for example, PDCP/RLC).

On the other hand, in step S1220, if the fast HO is not required, theprocess proceeds to step S1295, and thus terminal and the target basestation may perform the general handover procedure.

The handover method as described above eliminates the dependency of thecell in the ciphering and RRC reset operations and eliminates thetransmission/reception of unnecessary RRC messages to be able to improveperformance in terms of the mobility of the terminal and processing inthe cloud RAN (centralization & virtualization) environment.

FIG. 13 is a block diagram illustrating a configuration of a terminal1300 according to an embodiment of the present disclosure. The terminal1300 may include a transceiver 1310 and a controller 1320.

The transceiver 1310 is a component for transmitting and receiving asignal.

The controller 1320 is a component for controlling the terminal 1300 asa whole. Specifically, the controller 1320 may control the transceiver1310 to receive a message including the information on one or moretarget cell for performing the fast handover from the source cell.

The controller 1320 may control to perform the fast handover by reusingthe radio bearer based on the received information.

In addition, the controller 1320 may perform the fast handover byreusing the security key corresponding to the source cell and the radiobearer of the specific user plane protocol (for example, PDCP/RLC).

When the fast handover procedure is completed, the controller 1320 maycontrol the transceiver 1310 to transmit the handover complete messageincluding the new terminal identifier to the target cell. The securitykey corresponding to the source cell and the radio bearer of thespecific user plane protocol (for example, PDCP/RLC) are reused by thetarget cell that has received the handover complete message includingthe new terminal identifier.

On the other hand, the controller 1320 may control not only the abovecontents, but also the operations of the terminal 1300 in all of theabove embodiments.

FIG. 14 is a block diagram illustrating a configuration of a basestation 1400 according to an embodiment of the present disclosure. Thebase station 1400 may include a transceiver 1410 and a controller 1420.

The transceiver 1410 is a component for transmitting and receiving asignal.

The controller 1420 is a component for controlling the base station 1400as a whole.

First, the base station 1400 may perform the fast HO of the source celland the intra-target cell in the cell. At this time, the controller 1420may control the operation of each cell by being included in each of thesource cell and the target cell. However, this is only an example, andthe controller 1420 may be included in the base station 1400 to controlthe operations of the source cell and the target cell.

First, when the controller 1420 is a controller of the source cell orthe base station 1400, the controller 1420 controls the transceiver 1410to receive the measurement report including the measurement result ofone or more cell from the terminal. The controller 1420 may determineone or more target cell capable of performing the fast handover in whichthe radio bearer is reused, based on the measurement report.

For example, the controller 1420 may determine another cell in the basestation 1400 as the target cell performing the fast HO based on themeasurement report.

In addition, the controller 1420 may control the transceiver to transmitthe information on the one or more target cell determined to theterminal.

Meanwhile, at the time of performing the fast handover, the security keycorresponding to the source cell and the radio bearer of the specificuser plane protocol (e.g., PDCP/RLC) may be reused by the terminal andthe target cell.

In addition, when the controller 1420 is the controller of the basestation 1400, in the one or more target cell, the controller 1420 maycontrol the security key and the RB reset of the specific user planeprotocol (for example, PDCP/RLC) to be skipped when the target cellwhich performs the fast handover with the terminal is the cell of thebase station such as the source cell. Such an operation may also beperformed by the controller of the target cell when the controller ofthe target cell exists separately.

Meanwhile, if the target cell in which the fast handover is performed isthe cell of the base station different from the source cell and thecontroller 1420 is the controller of the source cell, the controller1420 may control the transceiver 1410 to transmit the information on thesecurity key and the radio bearer of the specific user plane protocol(for example, PDCP/RLC) to the target cell.

In the one or more target cell, when the target cell performing the fasthandover with the terminal is the cell of the base station differentfrom the source cell, the controller 1420 may control the transceiver1410 to transmit the sequence number status of the RLC to the targetcell when it is the controller of the source cell.

On the other hand, when the controller 1420 is the controller of thetarget cell, the controller 1420 may control the transceiver 1410 toreceive the handover message including the new terminal identifier fromthe terminal when the fast handover is completed and to reuse thesecurity key corresponding to the source cell and the radio bearer ofthe packet data convergence protocol (PDCP) and the radio link control(RLC) based on the new terminal identifier of the received message.

In addition, when the target cell is the cell of the base stationdifferent from the source cell, the controller 1420 may control thetransceiver 1410 to receive the information on the security key and theradio bearer of the PDCP/RLC and reuse the security key corresponding tothe source cell and the radio bearer of the PDCP/RLC based on thereceived information.

When the target cell is the cell of the base station different from thesource cell, the controller 1420 may control the transceiver 1410 toreceive the sequence number status of the RLC from the source cell.

When the source cell and the target cell are cells of different basestations, such operations may be performed by each of the controllers ofthe cells.

On the other hand, the controller 1420 may control not only the abovecontents, but also the operations of the base station 1400 in all of theabove embodiments.

The components of the terminal and the base station described above maybe implemented by software. For example, the controller of the terminaland the base station may further include a flash memory or othernonvolatile memories. In the nonvolatile memory, a program forperforming each role of the controller may be stored.

Further, the controller of the terminal and the base station may beimplemented in a form including a CPU (not illustrated) and a randomaccess memory (RAM). The CPU of the controller may copy theabove-described programs stored in the nonvolatile memory into the RAM,and then perform the copied programs to perform the functions of theterminal as described above.

The controller is a component serving to control the terminal and thebase station. The controller may be used in the same meaning as acentral processing unit, a microprocessor, a processor, an operatingsystem, or the like. Further, the controller of the terminal and thebase station may be implemented as a system-on-a-chip or a system onchip (SOC or SoC) along with other function sections such as thecommunication module included in the terminal.

Meanwhile, the method of the terminal and the base station according tovarious exemplary embodiments of the present disclosure as describedabove may be stored in a non-transitory computer readable medium whilebeing coded in software. The non-transitory computer readable medium maybe used while being equipped in various apparatuses.

The non-transitory computer readable medium is not a medium that storesdata therein for a while, such as a register, a cache, a memory, or thelike, but means a medium that semi-permanently stores data therein andis readable by a device. In detail, the non-transitory computer readablemedium may be a CD, a DVD, a hard disc, a Blueray disc, an USB, a memorycard, an ROM, etc.

Although the exemplary embodiments of the present disclosure have beenillustrated and described hereinabove, the present disclosure is notlimited to the above-mentioned specific exemplary embodiments, but maybe variously modified by those skilled in the art to which the presentdisclosure pertains without departing from the scope and spirit of thepresent disclosure as disclosed in the accompanying claims. Thesemodifications should also be understood to fall within the scope of thepresent disclosure.

1. A terminal in a wireless communication system, comprising: atransceiver configured to transmit/receive a signal; and a controllerconfigured to control a transceiver to receive a message includinginformation on one or more target cell for performing a fast handoverfrom a source cell and perform the fast handover reusing a radio bearerbased on the received information.
 2. The terminal of claim 1, whereinthe controller is further configured to performs the fast handover byreusing a security key corresponding to the source cell and radiobearers of service data adaptation protocol (SDAP), packet dataconvergence protocol (PDCP), a radio link control (RLC), a medium accesscontrol, or a physical layer.
 3. The terminal of claim 1, wherein thecontroller is further configured to control the transceiver to transmita handover complete message including a new terminal identifier to thetarget cell in case that the fast handover procedure is completed, andwherein the security key corresponding to the source cell and the radiobearer of the specific user plane are reused by the target cell thatreceives the handover complete message including the new terminalidentifier.
 4. A source cell in a wireless communication system,comprising: a transceiver configured to transmit/receive a signal; and acontroller configured to: control the transceiver to receive ameasurement report including a measurement result for one or more cellfrom a terminal, determine one or more target cell performing a fasthandover reusing a radio bearer based on the measurement report, andtransmit information on the one or more target cell determined to theterminal.
 5. The source cell of claim 4, wherein in case that the fasthandover is performed, a security key corresponding to the source celland radio bearers of service data adaptation protocol (SDAP), packetdata convergence protocol (PDCP), radio link control (RLC), mediumaccess control, or a physical layer are reused by the terminal and thetarget cell.
 6. The source cell of claim 4, wherein the controller isfurther controlled to control the transceiver to skip the security keyand an RB reset of the specific user plane in case that the target cellperforming the fast handover with the terminal is a cell of the basestation like the source cell in the one or more target cell and transmitinformation on the security key and the radio bearer of the specificuser plane to the target cell in case that the target cell performingthe fast handover is the cell of the base station different from thesource cell.
 7. The source cell of claim 4, wherein the controller isfurther configured to control the transceiver to transmit a sequencenumber status of the RLC to the target cell in case that the target cellperforming the fast handover with the terminal is the cell of the basestation different from the source cell, in the one or more target cell.8. A target cell in a wireless communication system, comprising: atransceiver configured to transmit/receive a signal; and a controllerconfigured to control the transceiver to receive a handover messageincluding a new terminal identifier from a terminal in case that a fasthandover is completed and reuse a security key corresponding to thesource cell and radio bearers of service data adaptation protocol(SDAP), packet data convergence protocol (PDCP), radio link control(RLC), a medium access control, or a physical layer based on the newterminal identifier of the received message.
 9. The target cell of claim8, wherein the controller is further controlled to control thetransceiver to receive information on the security key and a radiobearer of a specific user plane in case that the target cell is a cellof a base station different from the source cell, and reuse the securitykey corresponding to the source cell and the radio bearer of thespecific user plane based on the received information.
 10. The targetcell of claim 8, wherein the controller is further controlled to controlthe transceiver to receive a sequence number status of an RLC from thesource cell in case that the target cell is a cell of a base stationdifferent from the source cell.
 11. A handover method of a terminal in awireless communication system, comprising: receiving, from a sourcecell, a message including information on one or more target cell forperforming a fast handover; and performing the fast handover by reusinga radio bearer based on the received information.
 12. The handovermethod of claim 11, wherein the performing of the fast handover furtherincludes: performing the fast handover by reusing a security keycorresponding to the source cell and radio bearers of service dataadaptation protocol (SDAP), packet data convergence protocol (PDCP),radio link control (RLC), a medium access control, or a physical layer,and transmitting a handover complete message including a new terminalidentifier to the target cell in case that the fast handover procedureis completed, wherein the security key corresponding to the source celland the radio bearer of the specific user plane are reused by the targetcell that receives the handover complete message including the newterminal identifier.
 13. A handover method of a source cell in awireless communication system, comprising: receiving, from a terminal, ameasurement report including a measurement result for one or more cell;determining one or more target cell performing a fast handover reusing aradio bearer based on the measurement report; and transmittinginformation on the one or more target cell determined to a terminal. 14.The handover method of claim 13, wherein in case that the fast handoveris performed, a security key corresponding to the source cell and radiobearers of service data adaptation protocol (SDAP), packet dataconvergence protocol (PDCP), radio link control (RLC), a medium accesscontrol, or a physical layer are reused by the terminal and the targetcell, and the handover method further includes: transmitting a sequencenumber status of an RLC to the target cell in case that the target cellperforming the fast handover with the terminal is a cell of a basestation different from the source cell, in the one or more target cell;skipping the security key and an RB reset of a specific user plane incase that the target cell performing the fast handover with the terminalis the cell of the base station like the source cell, in the one or moretarget cell; and transmitting information on the security key and theradio bearer of the specific user plane to the target cell in case thatthe target cell performing the fast handover is the cell of the basestation different from the source cell.
 15. A handover method of atarget cell in a wireless communication system, comprising: receiving,from a terminal, a handover message including a new terminal identifierin case that a fast handover is completed; and reusing a security keycorresponding to a source cell and radio bearers of service dataadaptation protocol (SDAP), packet data convergence protocol (PDCP),radio link control (RLC), a medium access control, or a physical layerbased on the new terminal identifier of the received message.